The present invention relates to impingement cooling of a gas turbine nozzle segment side wall and particularly relates to cooling inserts for targeting impingement cooling flows along the nozzle side wall.
Land-based gas turbines are used for the generation of electrical power. In a current turbine design, certain parts of the rotor including the turbine buckets, as well as certain fixed parts including nozzle segments, are cooled by flowing a cooling medium, e.g., steam, through the parts. For example, nozzle segments each having outer and inner band portions with one or more vanes extending therebetween are arranged in a circular or annular array about the rotational axis of the turbine to define and form part of the hot gas path through the turbine. Generally, the outer band portion of each nozzle segment has a nozzle wall forming part of the hot gas path and a cover radially outwardly of the nozzle wall defining a cooling chamber. An impingement plate is disposed in the chamber defining first and second cavities on opposite sides of the impingement plate with the cover and nozzle wall, respectively. A cooling medium, e.g., steam, flows into the first cavity and through multiple apertures in the impingement plate for impingement cooling the nozzle wall. The post-impingement cooling medium flows from the second cavity into cavities within the vane(s) which typically have inserts for impingement cooling the side walls of the vane. The cooling medium then flows into the inner band portion of the nozzle segment which, like the outer band portion, is divided into first and second cavities. The flow from the vane cavities thus flows into a first cavity, and reverses direction for flow through apertures in an impingement plate for impingement cooling the inner nozzle wall. The spent cooling medium then flows outwardly through one or more cavities of the vane to a cooling medium exhaust.
In a current nozzle segment design, the cover is welded to the nozzle side walls or edges locating the weld joint close to the hot gas path. The weld joint is therefore subjected to the high temperatures of the flowing hot gas. In that design, nozzle side wall cooling is very difficult and the weld joint is a life-limiting location. Due to fabrication/assembly requirements of the impingement plate, a substantial distance is created in which the cooling medium must traverse to impingement cool the nozzle side wall or edge. This distance reduces the effectiveness of the impingement cooling at the side wall. Thus, the current weld joint between the nozzle side wall and its cover has reduced fatigue life due to the geometry of the location of the weld and the temperature at that location. It will also be appreciated that when providing a weld at that location, weld distortion can change the thickness of the weld joint. If the joint becomes too thin, structural strength is sacrificed. Moreover, for each weld joint, there is a reduction in cycle life. Thus, for welds of various thicknesses, the stresses increase, low cycle fatigue decreases and the life of the parts is severely limited. Also, the machining preparation for the weld joint at its current location is critical and the design is very sensitive to manufacturing tolerances. As indicated previously, impingement cooling of the band edge becomes ineffective and less than optimum because of the substantial distances traversed by the flow of the cooling medium from the impingement plate apertures to the side wall.
In a preferred embodiment of the present invention, a cooling system is provided in a nozzle segment wherein the weld joint between the side wall and cover is displaced radially away from the hot gas path to a location on the side of the spline seal between adjacent nozzle segments remote from the nozzle wall exposed to the hot gas path. That weld location assures that the weld thickness is not a variable and that the weld does not require subsequent machining. Moreover, by locating the weld a substantial distance from the hot gas path, the life of the weld joint and the producibility of the joint is vastly improved. It remains necessary, however, to cool the side wall of each nozzle segment. The cooling scheme of the present invention provides for the producibility of the complex cooling required at the band edges and reduces the distance between the impingement apertures and the side wall to provide effective side wall impingement cooling.
To accomplish the foregoing, the nozzle side wall has an inturned flange defining with the side wall and portion of the nozzle wall an undercut region. The cover is welded to the inturned flange of the nozzle segment at a location spaced from the hot gas path. For example, in the case of the outer band, the weld joint is located outwardly of the edge spline seal between adjacent nozzle segments. To provide impingement cooling for the nozzle side wall or edge, the impingement plate disposed in the chamber defined between the nozzle wall and the cover has an inturned edge which is welded to the inturned flange of the nozzle side wall. The undercut region defined by the inturned flange, the nozzle side wall, and a portion of the nozzle wall defining part of the hot gas path is of substantial volume. Moreover, the distances between the impingement plate and the walls defining the undercut region are sufficiently large that cooling flow through the apertures of the impingement plate has reduced effectiveness in the undercut region for cooling the side wall. To provide an effective impingement cooling flow in the undercut region for cooling the side wall, one or more openings are formed along the inturned edge of the impingement plate. Cooling inserts or receptacles are disposed in the openings and project into the undercut regions. Each receptacle has an open side which receives cooling medium flow from the first cavity between the cover and the impingement plate. The walls of the receptacle which project toward and lie in opposition to the nozzle side wall have apertures which are spaced sufficiently close to those walls to afford effective impingement cooling. Thus, the cooling medium flows from the first cavity, through the one or more openings of the inturned edge of the impingement plate, into the receptacle(s) and through the receptacle apertures for impingement cooling of the side wall and the nozzle wall portion in the undercut region. This affords effective cooling of the nozzle edge. An additional advantage of the use of receptacles resides in the flow of the spent cooling medium in flow areas between the inturned flange of the nozzle side wall and a wall of the receptacle toward a lower pressure area. This facilitates the flow of the cooling medium from the nozzle wall into the nozzle vane. It will be appreciated that the inner band portion of the nozzle segment has a similar cooling arrangement along its opposite side walls.
In a preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having a band portion including a nozzle wall defining in part a hot gas path through the turbine, a cover spaced from the nozzle wall defining a chamber therebetween and an impingement plate disposed in the chamber defining with the cover a first cavity on one side of the impingement plate for receiving a cooling medium, the impingement plate on an opposite side thereof defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures for flowing cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, the nozzle segment including a side wall including an inturned flange extending along the side wall defining with the side wall and nozzle wall an undercut region, and means projecting into the undercut region and toward the side wall for receiving cooling medium from the first cavity, the projecting means having a plurality of apertures therethrough for flowing cooling medium from the first cavity into the undercut region for impingement cooling a portion of the nozzle side wall.
In a further preferred embodiment according to the present invention, there is provided for use in a gas turbine, a nozzle segment having outer and inner band portions and at least one vane extending between the band portions, at least one of the band portions including a nozzle wall defining in part a hot gas path through the turbine, a cover radially spaced from the nozzle wall defining a chamber therebetween and an impingement plate disposed in the chamber defining with the cover a first cavity on one side of the impingement plate for receiving a cooling medium, the impingement plate on an opposite side thereof defining with the nozzle wall a second cavity, the impingement plate having a plurality of apertures for flowing cooling medium from the first cavity into the second cavity for impingement cooling the nozzle wall, the nozzle segment including a side wall extending generally radially between the nozzle wall and the cover and including an inturned flange defining an undercut region extending along the side wall and forming part of the second cavity, the impingement plate having an edge secured to the side wall and having at least one opening therethrough for communicating cooling medium from the first cavity into the undercut region, a receptacle open on one side disposed in the one opening for receiving cooling medium from the first cavity and extending into the undercut region toward the side wall, the receptacle having a plurality of apertures therethrough for flowing cooling medium received through the one opening into the undercut region for impingement cooling a portion of the nozzle side wall.